Another battery would work, except it would require my laptop to charge it and I don't take my laptop home, due to an issue there. But this would be easy to charge at home and use later.
You might be able to find external battery chargers for some laptop batteries.
When I used my PowerBook G3, I had bought an external charger that could charge the batteries outside the laptop.
Of course, before worrying about whether you can find such a best, the batteries would need to be removable (which is not true for many laptops these days).
And even assuming the batteries are removable and you can find an external charger, there is still the issue of whether or not you can swap the batteries without having to power down the computer. Not all computers can do that.
So, you are likely better off with an external battery pack like Animal